In recent years, semiconductors, including compound semiconductors, have been used in an increasingly wider range of application fields by taking advantage of various characteristic features of the semiconductors. For example, a compound semiconductor is useful as a base substrate for epitaxial layers to be laminated on, thus, the compound semiconductor is used in semiconductor devices such as a light-emitting diode and a laser diode.
If a semiconductor substrate is used as the base substrate, a surface of the semiconductor substrate needs to be processed into a strain-free mirror surface. Accordingly, preprocessing (for example, cutting, lapping and etching) is performed on a single-crystal semiconductor ingot to obtain a semiconductor substrate, and then mirror-polishing is performed on a surface of the semiconductor substrate.
As semiconductor substrates, there are known, for example, those described in Patent Literatures 1 to 3 below. Patent Literature 1 discloses a semiconductor substrate obtained by cutting a crystalline III-V nitride (for example, (Al, Ga, In)—N) crystal-grown by means of vapor-phase epitaxy (VPE) and then performing preprocessing. Patent Literature 1 discloses performing chemical polishing (CMP) after mechanically polishing a surface of the semiconductor substrate, as a preprocessing, in order to remove superficial damage caused by the mechanical polishing.
Patent Literature 2 discloses a semiconductor substrate in which a surface of an AlxGayInzN (0<y≦1, x+y+z=1) wafer is polished by CMP to reduce RMS-based surface roughness to less than 0.15 nm, thereby reducing surface defect and contamination. Patent Literature 2 discloses using Al2O3 or SiO2 as abrasive grains at the time of performing CMP and adjusting pH by adding an oxidizing agent to a polishing liquid.
Patent Literature 3 discloses a semiconductor substrate in which an Si concentration at a boundary face between an epitaxial layer and the semiconductor substrate is set to 8×1017 cm−3 or lower, on the assumption that Si piled up (accumulated) on the boundary face between the epitaxial layer and the semiconductor substrate deteriorates device characteristics.